Catalytic oxydehydrogenation process

ABSTRACT

Isobutyric acid or a functional equivalent; e.g., a lower alkyl ester is oxidatively dehydrogenated to effect the vapor phase conversion thereof to the corresponding α,β-ethylenically unsaturated derivative by contact with a heterogeneous catalyst in the presence of molecular oxygen. The catalyst is composed of calcined phosphates of iron containing cobalt or lanthanum as a modifier or dopant component.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a process for the conversion of isobutyricacid to methacrylic acid including the like conversion of a lower alkylester of isobutyric acid.

2. Description of the Prior Art

There is considerable prior art directed to the oxydehydrogenation ofthe lower saturated mono-carboxylic acids to prepare the corresponding,α,β-ethylenically unsaturated acids. The initial work reported in thisarea was that of thermally effecting the indicated oxydehydrogenation bythe vapor phase reaction of the acid substrate with iodine and oxygen.This approach has not attracted much attention as a potentially viableway for commercially implementing the underlying reaction. This isunderstandably so inasmuch as iodine is costly, exhibits exttremecorrosivity properties and poses considerable problems in realizingcomplete recovery of the comparatively large amounts thereof required inthe process.

As the subsequent prior art picture amply points up, the heterogeneouscatalytic method for effecting the oxydehydrogenation reaction is viewedas being much more attractive from the standpoint of potentialcommercial applicability. In the main, the more recent relevant priorart activities have centered on the use of two types of catalystcompositions for this purpose. One type includes generally theheteropoly-acids, typically representative of which 12-molybdophosphateoptionally including vanadium and/or tungsten elements in a likestructural arrangement. The other type catalyst includes those systemshaving in common a calcined iron phosphate matrix.

Iron phosphate subjected to calcination exists in a plurality ofcrystalline phases or species. While it is believed that theoxidation/reduction couple involved in the underlying reaction isattributable to the iron phosphate, which species is or arecatalytically active has not been identified. There is, however,evidence that the presence of an extrinsic metal component in thepreparatory serves to facilitate the formation of the catalyticallyactive species. For example, U.S. Pat. No. 3,948,959 notably teachesthat an alkali or alkaline earth metal as the extrinsic metal componentis effective for this purpose. The present invention accordinglyrepresents a furtherance of this particular aspect of the current stateof the art.

SUMMARY OF THE INVENTION

In accordance with this invention, a catalytic process is provided foreffecting the oxidative dehydrogenation of isobutyric acid or a loweralkyl ester thereof to form the corresponding α,β-ethylenicallyunsaturated derivative. The process of this invention comprisescontacting a heterogeneous catalyst at a temperature from 300°-500° C.with a co-feed of said substrate and diluted oxygen gas, characterizedin that said catalyst is calcined iron phosphate containing theextrinsic metal cobalt or lanthanum as a modifier or dopant component.In the broadest aspect of the invention the contemplated catalyst isdefined by the gram-atom empirical formula FeP₁₋₂ M₀.01-1 O_(x) in whichM represents cobalt or lanthanum and x represents the number of oxygenatoms bound to the other metals in their respective states of oxidationin the catalyst.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

There are a number of techniques applicable for preparing the catalystuseful in the practice of this invention. Of these, the more facilemethods involve preparing the integral composition prior to calcination.This can be readily accomplished by employing the so-called slurrymethod or the precipitation method. In the latter method an aqueoussolution of salts of the contemplated metals and phosphoric acid isfirst prepared and thereupon neutralized with an appropriate base inorder to precipitate the mixed metal phosphates. The precipitate isdesirably carefully washed to remove all traces of water solubles andthen dried prior to calcining. In the alternative, one can add ammoniumphosphate to the solution of the metal salts in order to precipitatedirectly the metal phosphates. As indicated, any water-soluble salt ofiron or the dopant metal can be used. However, because of the solubilitycharacteristics of the nitrate salts, among other reasons, such saltsare preferred.

The so-called slurry method is even more convenient to carry out and forthis reason represents the preferred method herein. In accordance withthis procedure, the aqueous solution of the iron and dopant metal saltstogether with the phosphoric acid is obtained as previously noted.However, no precipitation is effected as the solution is subjected toheating with stirring in order to remove water. Heating is continueduntil the mass can be no longer stirred. The residue is then fragmentedand again heated at a moderately elevated temperature in the order ofabout 120° C. until completely dried. Thereupon the dried composite issized and calcined. Suitable calcination temperatures broadly range from400°-450° C. Applicable calcination periods range from 2-16 hours.

In the manner of either of these techniques, a supported catalyst can beprepared. For example, in the slurry method colloidal silica or anyother form thereof as well as other supports such as alumina, quartz,titanium dioxide, etc., can be added prior to removing the watercontent. Similarly, in the alternate method described, the precipitationof the indicated compounds can be accomplished in the presence ofsuspended particulates of the intended support.

The catalyst compositions of this invention can be employed in afluidized, stirred tank reactor, or fixed-bed type reactor. Because ofthe convenience associated with the use of a fixed-bed reactor in asmall scale operation, such a reactor will be exemplified herein. In thepreferred mode of operation the feed to the reactor comprises apre-heated gaseous mixture of the substrate, molecular oxygen, steam andinert diluent gas. A pre-heat temperature in the range of about 300° to350° C. is customarily observed. A broad range of applicable reactiontemperatures is from 300°-500° C. but more generally a temperature offrom 375° to 425° C. provides for optimum processing.

The mole ratio of molecular oxygen to substrate is from 0.5 to 1.5 andmore preferably from 0.7 to 0.75 in the case where the substrate isisobutyric acid per se. While steam is not necessary for the purpose ofeffecting the reaction, the presence thereof in the feed is particularlydesirable insofar as it acts as a heat sink and in such a capacityserves to minimize combustion of the substrate. An applicable mole ratioof water to the substrate in the feed is from about 8-20. Optimum ratiois more in the order of from 12 to 15.

Another important parameter resides in the concentration of thesubstrate in the feed. Expressed in terms of mole percents, theconcentration of the contemplated substrates ranges broadly from 0.1-20.As is common in reactions of this type, yield of the desired product isan inverse function of the concentration. From the standpoint ofachieving a reasonable through-put combined with an acceptable yield,the concentration of the substrate in the feed is from about 3-6 molepercent. Concentration is controlled by the amount of the inert gaspresent in the feed stream. The preferred diluent gas is nitrogenalthough other gases such as carbon dioxide, helium, argon, and the likeare suitable. Of course if the desired concentration of substratepermits, air represents a suitable diluted oxidant.

A further important parameter is that of contact time. Contact orreaction time is defined as the catalyst volume divided by the volume ofgas feed per second at reaction temperature. The catalyst volume is thebulk volume occupied by the catalyst in the reactor. The term catalystin this sense not only includes the modified iron phosphate itself butalso the support if present. Accordingly, applicable reaction time rangefrom 0.05-3.0 seconds and more generally in the order of from 0.1-1.0second. The reaction is carried out preferably at atmospheric pressurealthough a higher pressure up to about 10 atmospheres is applicable.

EXAMPLE I

The purpose of this example is to illustrate the hereinabove describedslurry method for preparing a supported lanthanum doped iron phosphatecatalyst useful in the practice of this invention.

Iron nitrate nonahydrate in the amount of 404.0 g. along with 86.2 g. oflanthanum nitrate 0.6H₂ O and 140 g. of 85% phosphoric acid weredissolved in 400 ml of distilled water. The solution of metal salts andacid was mixed with 100 ml of LUDOX 40 HS and stirring continued at 85°C. until the bulk of the water had been evaporated. The resultant pastewas further dried at 120° C. until in condition to be sized whereupondrying was continued for 6 hours. The dried particulates were thencalcined at 450° C. for 6 hours. The gram-atom empirical formula of thecalcined composition follows: FeP₁.44 La₀.2 O_(x) /20% SiO₂.

EXAMPLE II

In a manner similar to that described in Example I a cobalt doped ironphosphate catalyst composition was prepared. The gram-atom empiricalformula of the resultant compositions follows: FeP₁.44 Co₀.1 O_(x) /3.5%SiO₂.

EXAMPLE III

This example illustrates the use of the catalyst compositions of theforegoing examples in effecting the oxidative dehydrogenation ofisobutyric acid. The reactor and the general manner of conducting thereaction was the same for the enumerated runs. The procedure observedconsisted of feeding a pre-heated mixture of the isobutyric acid,oxygen, nitrogen and steam through a stainless steel tube of 1/2" OD(3/8" ID) and approximately 18" in length containing a 15 cc. bed of thetest catalyst maintained at the reaction temperature utilized in theparticular run.

The pre-heater consisted of a length of stainless steel tube similar tothe reactor but packed with glass beads. Any carbon dioxide formed inthe course of reaction was absorbed in an Ascarite tube protected by acalcium sulfate absorber for any uncondensed water. The condensedorganic product was separated from the water, collected and analyzed bythe internal standard method of gas chromotography.

Other pertinent processing conditions observed for the individual runsare set forth in Table 1 presented hereinbelow. The results obtained interms of selectivity and conversion are likewise given in said table.Conversion represents the mole ratio of substrate consumed to thatcharged to the reactor. Selectivity to methacrylic acid represents themole ratio of methacrylic acid found in the effluent to that of IBAconsumed in the reaction.

                                      TABLE I                                     __________________________________________________________________________    OXYDEHYDROGENATION OF ISOBUTYRIC ACID (IBA)                                                        CHARGE TO REACTOR                                                        REAC-                            TOTAL                                                                              CON-                                    TION O.sub.2 /IBA                                                                        IBA                   GAS  VER-                                                                              SELEC-              RUN        TEMP.                                                                              TIME (Mole Conc. IBA FEED                                                                              WATER   FEED SION                                                                              TIVITY              NO. CATALYST                                                                             (°C.)                                                                       (Sec.)                                                                             Ratio)                                                                              (Mole %)                                                                            (gm/hr @ RT)                                                                          (gm/hr @ RT)                                                                          (1/hr)                                                                             (%) (%)                 __________________________________________________________________________    1   Ex. I* 406  0.23 0.66  4.4   8.5     27.0    53.4 91.5                                                                              63.9                2   "      380  0.22 0.65  4.1   9.36    31.9    55.4 87.3                                                                              69.6                3   Ex. II**                                                                             410  0.56 0.50  5.0   8.8     20.0    41.5 74.0                                                                              62.0                4   "      410  0.56 0.68  5.0   8.8     20.0    41.6 99.0                                                                              53.0                __________________________________________________________________________     *15 cc. packed bed (50% supported catalyst50% quartz)                         **15 cc. packed bed of indicated catalyst                                

What is claimed is:
 1. In a process for the catalytic conversion ofisobutyric acid or a lower alkyl ester thereof to the correspondingα,β-ethylenically unsaturated derivative via the oxydehydrogenationreaction wherein an iron phosphate catalyst is contacted with a gaseousfeed stream containing said acid or ester substrate and oxygen at atemperature between about 300° and 500° C.; the improvement of effectingsaid oxygehydrogenation reaction in the presence of a modified ironphosphate catalyst having the gram-atom empirical formula FeP₁₋₂M₀.01-1^(O) _(x) in which M represents cobalt or lanthanum and xrepresents the number of oxygen atoms bound to the other elements intheir respective states of oxidation in the catalyst.
 2. The process inaccordance with claim 1 wherein the substrate is isobutyric acid.
 3. Theprocess in accordance with claim 2 wherein M is lanthanum.
 4. Theprocess in accordance with claim 2 wherein M is cobalt.